Display an AJAX tree from your DB using CTreeView

Tutorials

AJAX, CTreeView

## The database
 
 
This example uses a MySQL DB with a table named `tree` with the fields `id`, `name`, and `parent_id`. The `parent_id` will be `NULL` for root elements. The SQL is kept simple (no autoinc, no FK, etc).

~~~~MySQL has no recursive queries. In order to fetch tree nodes, you have to recursively send SQL queries for each node, asking for its children. The easiest way to do so is to send these queries with AJAX, so that the tree can be displayed even if the deep nodes haven't been fetched yet. If you're using Oracle or Postgresql, there are other solutions, though this will also work.

## The view
 
 
In your view, add:

~```~

This will create a tree widget [CTreeView](http://www.yiiframework.com/doc/api/1.1/CTreeView/) on your page, and its data will be fetched using the `ajaxFillTree` of the current controller. ## The controller
 
 
So we have to add this action to the controller:

~public function actionAjaxFillTree()
{
// accept only AJAX request (comment this when debugging)
 
if (!Yii::app()->request->isAjaxRequest) { exit(); } // parse the user input
 
$parentId = "NULL"; if (isset($_GET['root']) && $_GET['root'] !== 'source') { $parentId = (int) $_GET['root']; } $// read the data (this could be in a model)
 
$childreqn = Yii::app()->db->createCommand( "SELECT m1.id, m1.name AS text, m2.id IS NOT NULL AS hasChildren " . "FROM tree AS m1 LEFT JOIN tree AS m2 ON m1.id=m2.parent_id " . "WHERE m1.parent_id <=> $parentId " . "GROUP BY m1.id ORDER BY m1.name ASC" );
 
$children = $req->queryAll();
echo str_replace(
'"hasChildren":"0"',CTreeView::saveDataAsJson($children)
);
exit();
 
}
```~When sending the JSON-encoded result, there's a little trick: the javascript wants the `hasChildren` attribute to be a boolean. But the SQL result just contains 0 or 1, so we have to convert it, at least for the "false" case. Instead of operating on the string result, one could modify the PHP array with a `foreach`.

The `exit()` might not be necessary, but if you enabled the logs to Firebug or extensions like a debug toolbar, then Yii will write some JS at the end of your response, and it will break the JSON format.
 
### If AJAX fails
 
 
If your AJAX query fails, you should try to debug it in your browser. Firefox, Chrome and Opera have good developer toolbars that will show you the AJAX requests sent and what their answers were. You may find that some HTML is appended at the end of the expected JSON answer. I had this problem with Yii'JS logs and a toolbar extension. In this case, insert an `exit();` at the end of the action. Or even better: `Yii::logger->flush(); exit();`.
 
 
### Final note
 
 
This is meant as a simple introduction. There are many ways to enhance this:
 
 
* Refactor the controller action by putting its 2 last lines into a model.
 
* Increase the abstraction level, so that the code can be used on various models, as long as they share the same table structure.
 
* If the tree content rarely changes, you may consider using a different structure, like nested sets. This will make most queries easier, e.g. fetching the path of a node.